In the last Astronomy Packet we talked about how
scientists study the Earth to understand other worlds. They
also study worlds which are similar to each other in order to better
understand how these worlds work. Comparing different planets
to the Earth and to each other is called Comparative
Planetology. In Astronomy Packet # 2 we learned about the Moon
and Mercury. We used comparative planetology to understand how
they are the same and how they are different. This week we are
going to explore the following planets: Venus, Earth and Mars.

Ecosphere:

In order for life as we know it to exist there must
be liquid water. Around every star there is a zone where the
temperature is just right for water to remain as a liquid. If
the planet is too close to the star, it will be too hot and all its
water will evaporate. If the planet is too far from the star,
then it will be too cold and so all the water will freeze. But
if the planet sits just the right distance from its star, then it will
be just the right temperature for the water to remain liquid.
This life zone is called the Ecosphere.

The Earth sits almost right in the middle of the
Sun's Ecosphere, or life zone. But guess what? Venus and
Mars also sit in the Ecosphere, but Venus is on the very inner
edge, and Mars is on the very outer edge. This is the first
thing these three planets have in common. All three are within
the Sun's Ecosphere.